1. Field of the Invention
The present invention relates to a pneumatic tire provided with a block in a tread surface, and is particularly useful as a studless tire.
2. Description of the Related Art
Conventionally, in the studless tire, a cut called as a sipe is formed in a block of a tread surface, and a traveling performance on an iced road surface having a low coefficient of friction is enhanced based on an edge effect and a drainage effect generated by the sipe. As the sipe mentioned above, a linear sipe and a waveform sipe are put to practical use, the linear sipe extending linearly along a longitudinal direction, and the waveform sipe extending like a wavy form or a zigzag form.
The inventor of the present invention pays attention to a fact that a motion of the block is great on the iced road surface, and a ground contact pressure within the block tends to be uneven due to the motion, for further enhancing the traveling performance on the iced road surface. Further, as a result of repeatedly making a study, it is found that a region in which the ground contact pressure becomes particularly high exists in the vicinity of a wall surface of the block, thereby enhancing the unevenness of the ground contact pressure and deteriorating the traveling performance on the iced road surface. A description will be given below of a knowledge relating to the ground contact pressure distribution mentioned above.
FIG. 8 shows a ground contact pressure distribution within a block at a time of applying a braking force to a tire on an iced road surface, and an arrow R shows a turning direction of the tire. In accordance with this drawing, it is known that the ground contact pressure is high in a region close to a wall surface 21 forming a front side in the turning direction, thereby forming a factor enhancing an unevenness of the ground contact pressure within the block 20. Further, the region having the high ground contact pressure exists near wall surfaces 22 and 23, and in the case that a lateral force generated by a turning travel motion is applied, the ground contact pressure in this region becomes further higher. In this case, at a time of applying a driving force to the tire, an upside down tendency to FIG. 8 is generated, and the ground contact pressure becomes higher in the region close to a wall surface 24 of the block 20.
Here, in Japanese Unexamined Patent Publication No. 2004-42773, there is described a tire in which a narrow groove extending in a tire diametrical direction is formed on a wall surface around a block. However, the tire is provided for removing a water film generated between the wall surface of the block and a snow ice, and does not suggest means for solving the unevenness of the ground contact pressure within the block mentioned above. Further, since the sipe on the upper surface and the narrow groove on the wall surface are in parallel to each other, in this tire, there is a problem that a rigidity of the block is locally and extremely lowered too much and a steering stability performance is deteriorated, at a position where they are arranged within the same plane.
Further, in Japanese Unexamined Patent Publication No. 2001-121926, there is described a tire in which a step portion is integrally provided around a block, and a sipe extending in a tire diametrical direction is formed in a side wall of the step portion. However, the tire is provided for bringing the sipe formed in the side wall of the step portion into sight on the tread surface so as to enhance an edge effect, for securing a snow traction performance after medium and end stages of a wear, and does not suggest means for solving the unevenness of the ground contact pressure within the block. Since a rigidity of a peripheral portion of the block is improved by the step portion, the ground contact pressure in the region close to the wall surface tends to come up, and there is a risk of promoting the unevenness of the ground contact pressure within the block.
Further, in Japanese Unexamined Patent Publication No. 2000-158915, there is described a tire in which a wall surface of a block is provided with a plurality of reinforcing ribs extending approximately in parallel to a tread surface and protruding into a groove. However, the tire is provided for suppressing a lift of the block from the road surface caused by a bending deformation, and does not suggest means for solving the unevenness of the ground contact pressure within the block mentioned above. Japanese Unexamined Patent Publication No. 2000-158915 defines that the rigidity with respect to a shearing direction which are in parallel to the road surface and a compressing direction is not changed by forming the wall surface of the block as an accordion shape by the reinforcing rib.